The following system is known from Japanese Patent Application Publication No. 2007-270783. Specifically, when a rider of a motorcycle rotates a throttle grip, the system causes an electric throttle valve to directly operate, not on the basis of a mechanical movement of a cable connected to a throttle grip, but on the basis of an electric signal from a sensor which electrically detects a rotation amount of the throttle grip. The system also automatically enables a running speed to be kept constant by an automatic cruise control device without the rider's throttle operation.